A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC--DC converters are frequently employed to advantage. DC--DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device, such as a field-effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC--DC converter.
Conventionally, the rectifier includes passive rectifying devices, such as Schottky diodes, that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, generally cannot achieve forward voltage drops of less than about 0.35 volts, thereby substantially limiting a conversion efficiency of the DC--DC converter. To achieve an acceptable level of efficiency, DC--DC converters that provide low output-voltages (e.g., 1 volt) often require rectifying devices that have forward voltage drops of less than about 0.1 volts. The DC--DC converters, therefore, generally use synchronous rectifiers. A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with rectifier switches, such as FETs or other controllable switches, that are periodically driven into conduction and non-conduction modes in synchronism with the periodic waveform of the AC voltage. The rectifier switches exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifying devices.
One difficulty with using a rectifier switch (e.g., an n-channel silicon FET) is the need to provide a drive signal that alternates between a positive voltage to drive the device into the conduction mode and a zero or negative voltage to drive the device into the non-conduction mode. Of course, depending on the type of rectifier switch, an opposite drive polarity may be employed. Although a capacitive charge within the rectifier switch may only be 30 to 50 nanocoulombs per device (rectifier switch), in situations where as many as a dozen or more devices may be used, a high drive current may be required for a brief period of time to change conduction modes.
The power required by the process of charging the control terminal(s) of the rectifier switch(s) (gate terminal, in the case of a FET) may be represented as the drive bias voltage multiplied by the total gate charge multiplied by the switching frequency and divided by the efficiency of the bias energy source employed. The power required may, for example, be equivalent to: 50*10.sup.-9 coulombs.times.8 volts.times.500,000 Hz.times.12 devices/0.8 bias efficiency=3 watts. In addition, typical drive currents may be 10 amperes or greater, lasting for tens of nanoseconds. The need to provide substantial power to the rectifier switch(s) to change conduction modes thus reduces some of the advantages of the synchronous rectifier.
Accordingly, what is needed in the art is a drive circuit for driving the rectifier switch of a synchronous rectifier that overcomes the deficiencies of the prior art.